1. Field of the Invention
The present invention relates to a process for preparing a glycerol from an aqueous crude glycerol solution containing impurities such as diols.
2. Discussion of the Related Art
One process for preparing a glycerol includes a process that goes through a step of transesterification, hydrolysis or saponification of an oil or fat. In a glycerol-containing solution obtained through esterification or others, there usually exist impurities such as an oily component, an alkali component, a soap component, a salt or diols. In the conventional process, a crude glycerol is obtained by either concentrating the glycerol-containing solution after subjecting the solution to filtration, distillation, treatment with active charcoal, ion-exchange treatment or the like; or concentrating the glycerol-containing solution, and thereafter subjecting the concentrate to treatment with active charcoal, ion-exchange treatment or the like.
A greater part of impurities such as an oily component can be removed by treatments such as filtration. However, a part of impurities, particularly diols of which physical properties are similar to those of the glycerol, such as 1,2-propanediol, 1,3-propanediol, 3-methoxy-1,2-propanediol, or 2-methoxy-1,3-propanediol, cannot be sufficiently removed by these treatments. In addition, the step of concentrating the glycerol-containing solution is purposed for separating water of which physical properties are significantly different from those of the glycerol. Therefore, it would be impossible to reduce the content of the diol respectively to 0.1% by weight or lower in the conventional concentrating step.
In order to solve these problems and to prepare a glycerol having high quality, various studies have been made as disclosed, for instance, in Japanese Patent Laid-Open Nos. Sho 55-157525, Sho 60-109534, Sho 61-140532 and Hei 1-135735.
Japanese Patent Laid-Open No. Sho 55-157525 discloses a process for suppressing an odor by adding a metal compound when a glycerol-containing solution is concentrated. In this process, however, a step is required for removing the added metal compound after the concentration. In addition, a high purity for the glycerol cannot be expected.
Japanese Patent Laid-Open No. Sho 60-109534 discloses a process comprising subjecting a glycerol-containing solution to distillation procedures under reduced pressure of 10 to 20 mbar and at a temperature of from 165.degree. to 180.degree. C. to previously remove the salt, thereafter isolating water with higher boiling impurities by a distillation tower, and purifying the obtained glycerol by a treatment with active charcoal. In this process, however, the process is carried out at a temperature as high as from 165.degree. to 180.degree. C. for removal of salts in the presence of impurities such as an oily component and others. Therefore, color and odor of a glycerol are deteriorated, and consequently, the treatment with active charcoal is undesirably required in the final step. Moreover, since the glycerol after concentration has a high viscosity, it would be extremely difficult to carry out the treatment with active charcoal. Furthermore, also in the rectification treatment, the fact that water is contained in the raw material as a non-condensing component in the tower in an amount of up to 10% by weight renders marked difficulty in the design for vacuum equipment for operating under the indicated degree of vacuum (5 to 10 mbar), whereby an uneconomical equipment design has to be made.
Japanese Patent Laid-Open No. Sho 61-140532 discloses a process comprising adding pentanol to a glycerol-containing solution to remove salts, and thereafter subjecting the glycerol-containing solution to flash distillation twice using a distillation tower, to give a purified glycerol. When the process is applied to the preparation of a glycerol derived from natural source, however, treatments are carried out at very high temperatures such that the temperature in each flash tower is 150.degree. and 170.degree. C., respectively, and that the temperature in the distillation tower is 190.degree. C., so that the deterioration in quality of the glycerol would be unavoidable. In addition, this publication does not disclose operating conditions in the distillation tower for removal of impurities.
Japanese Patent Laid-Open No. Hei 1-135735 discloses a process comprising adding a polycarboxylic acid to a crude glycerol, obtained by concentrating an aqueous glycerol solution to a concentration of 80 to 95% by weight, and distilling the mixed solution. According to this process, a purified glycerol having excellent stabilities in color and odor is prepared. In the process, however, since conditions for the distillation in the process are reduced pressure of from 10 to 20 mmHg and a temperature of from 150.degree. to 170.degree. C., there is a possibility of insufficient degree of vacuum or insufficient temperature for the removal of impurities by distillation. In addition, a treatment with active charcoal may be necessitated in a subsequent step, depending upon the amount of the polycarboxylic acid.
Furthermore, there arise important problems that a method of reuse of water generated in the course of concentration and a treatment method of water as wastewater needs to be studied, and that the yield of a glycerol needs to be improved, from the viewpoint of reduction in costs in the overall process for preparation.
An object of the present invention is to provide a process for preparing a glycerol from a crude glycerol comprising a diol and water, wherein the color and odor can be improved to an extent that post-treatments such as treatment with active charcoal or ion-exchange treatment is unnecessary after the distillation procedure, and individual impurities which are contained in the prepared glycerol can be remarkably reduced.
Another object of the present invention is to provide a process for preparing a glycerol wherein the recovered water is highly purified to an extent that an additional purification process is unnecessary, in a case where the recovered water is reused or treated as a wastewater in the process for preparing a glycerol.
A still another object of the present invention is to provide a process for preparing a glycerol wherein the glycerol-loss amount in the process for preparing a glycerol is reduced.
These and other objects of the present invention will be apparent from the following description.